I've been through this a few years back, when the promotion was $10 a month, and I had to call the first three to four months to adjust the bill, but after that I had $10 a month service for 4 full years. I thought they must've forgotten about me, but last year when I received more than $10 a month, I cancelled the internet sevices immediately.

Last month a dude from AT&T called and offered $14.99 a month service and I signed up, and today I receive $30 for the internet!!!

I am guessing it's like mail in rebate thing. They overcharge many thousands of customers, and only few hundred who have free time, patience call AT&T many times  to adjust their bills. And the rest who don't care,have no time to call, have no patience just pay what's on the bill. At the end AT&T makes a lot of money.

Just keep calling them, that's my advice, which I am going to do.